After playing a little bit with the example that Chris provided in his
last message, I have been able to embed the video in a real
presentation. Thanks for your help, Chris.

Since the corrected version pointed to the same error spotted by Ricardo
Pedroso, I owe him an apology. Sorry, I tried your examples and tinkered
with them during two hours (checking that with different variables and
trying to embed in the final presentation), but yesterday I was too
tired to realize that I was doing it wrong (I'd promise I was using the
same script yesterday than today [but it is too late for that ;-)]).

Thanks to both, Chris and Ricardo. And sorry guys for not realizing the
error before.